Shouldn't I be able to go to a club by myself when I'm 21?

heck yeah! if you are 21, why did you tell your mother, who is clearly overprotective? 2nd, if you live with your mom, it's time to get a life. she seems jealous that she can't be part of the birthday, so just take her out for dinner to IHOP but insist on clubbing alone, take a stand. you have been an adult for almost 3 years!